Add methods to collapse a window

We should add two methods to `egui::Window`

* `default_collapsed`: set whether the initial state of the window is collapsed
* `collapsed`: set whether the current state of the window is collapsed

Start at the egui::Window API and trace how a window's initial and current state are represented. The work is done when Window exposes default_collapsed for the initial state and collapsed for the current state, with behavior matching the issue description.

Written by the indexing model from the issue text.